Galling is a severe form of scuffing associated with gross damage to the mating surfaces the result of which is material transfer and seizure and is typically experienced in situations with the following:                (a) high metallurgical compatibility of mating surfaces (similar material compositions)        (b) high loadings between the surfaces        
Metal to metal contact points, and in particular threaded metal-metal contact points may be subject to galling during the making and subsequent breaking of the joint. In the oil industry, metal-metal connections, such as those between the individual pipe connections in completion strings, often require threaded metal-metal joints to be formed, which must withstand high temperatures and pressures, both internal and external, whilst remaining sealed. These seals must be easy to assemble and disassemble, for ease of use and safety. It is known to use anti-seize compounds to prevent galling, which operate by placing a dissimilar metal, such as copper, or metal-containing film between two similar metal surfaces. The dissimilar film thereby prevents the surfaces contacting directly, which could lead to fusion of the connection. Fused downhole connections could lead to failure of the tubing, and at the very least, galling and wear of the connection when it is dismantled.
In the past, in the oil industry, drilling operations have used an oilfield thread compound (also known as “thread dope” or “dope”), which is a grease/lubricant that has been formulated in accordance with American Petroleum Industry (API) bulletin 5A3. This compound is a substance that is applied to threaded oil field pipe connections, to provide lubrication during assembly and disassembly, and further to aid in sealing against the high internal and external pressures. As an example, the female mating component of steel alloy with a chromium content of greater than or equal to 9% is copper plated, and the dope is applied prior to make-up, of the connection. Such doping compounds typically contain heavy metals, such as antimony and lead, which can be toxic and harmful to the environment.
Legislation in certain countries, particularly in Europe, requires that only environmentally friendly lubricants should be used for oil and gas activity. Additionally, in Europe, there is a goal of achieving zero emission in oil field installations. Furthermore, it is likely that such legislation will spread to other countries, such as the United States of America, as environmental issues regarding oil exploration become more prominent. Environmentally friendly lubricants do exist (see for instance U.S. Pat. No. 5,536,422), but it is questionable whether they are as effective as the traditional, heavy metal-based systems.
Current API and API-modified dopes are not user friendly. They are difficult to apply evenly around the threaded surface areas, and are extremely difficult to remove from skin and work overalls. Due to the difficulties in applying such dopes (they are “thick” and tacky), they also tend to be used in quantities that generate significant amounts of waste. During the assembly of each connection, excess dope is forced into the bore of the pipe, and onto the external surface of the pipe. The longer the pipestring, the more connections are required, and therefore the more dope is forced into the pipestring, increasing the risk of well contamination. External surplus dope becomes a serious slip and contamination hazard, which is dangerous to those working in and around such areas. Such pipes can be several miles in length, so there can be a considerable amount of waste dope internally and externally.
GB2195939A relates to an extension drill rod for percussion drilling machines. The drill rod includes a wear lining in the form of a layer which may be copper bronze, aluminium bronze or another material softer than steel. The main reason for the choice of these materials is their softness relative to steel. The purpose of this coating is to combat fretting wear or erosion during use. There is no disclosure of the composition of the bronze, but given the requirement for a soft coating, it is to be expected that the coating would be high in copper or tin content.
It is known to provide a metal-to-metal sealing portion on tubular joints. It is also known for such sealing portions to include a coating. The joints may also include a threaded portion which has been coated. In GB2104919A there is described a coating which comprises a first activating layer and a second electrodeposited layer. The electrodeposited layer may be a layer of metal selected from the group consisting of iron, copper, zinc, chromium, nickel or an alloy containing at least one of these metals. There is, however, no indication that any one of these metals or alloys provides superior anti-galling properties.